There are far more boulders on the surface of asteroids than astronomers can account for. Now a team of astrophysicists has worked out why.

When Japan’s Hayabusa spacecraft gently manoeuvred into a parking orbit around the asteroid Itokawa in September 2005, its goal was to send a lander down to the surface to collect samples and bring them back to Earth, a mission it completed in June 2010. The spacecraft also conducted a comprehensive photographic survey of Itokawa, the most detailed ever taken of an asteroid.

These images have thrown up something of a surprise for astronomers. The surface of Itokawa is covered in relatively large boulders that look like ejecta from craters in other parts of the asteroid. But when astronomers added up the total volume of these boulders, it turned out to be greater than the volume of the craters they were supposed to have come from.

That left astrophysicists scratching their heads. The distribution of rock sizes on planets and moons follows a specific power law: there are a relatively tiny number of large boulders but huge numbers of small ones. Itokawa doesn’t follow this rule. The disproportionately large number of huge boulders is a mystery.

One of the few other asteroids photographed in similar detail is a near-Earth object called Eros. This was studied by the NEAR-Shoemaker probe which landed on the surface in 2001. It too has the same skewed distribution of large boulders over small ones.

All this raises an important question for astronomers. Why are there so many large boulders on the surface of these asteroids? What kind of process could be responsible?

Today, we get an answer thanks to the work of Soko Matsumura at the University of Dundee in Scotland and a few pals. These guys have pinpointed a mechanism that can explain the presence of the large boulders: the Brazil nut effect in which large particles in a sea of smaller ones rise to the surface when they are shaken. And the team have carried out simulations to confirm that the Brazil nut process does indeed have this effect.

First some background. Physicists have known for many years that granular materials can behave like fluids when shaken. Perhaps the most dramatic example of this occurs during earthquakes when areas of seemingly solid ground become fluid-like during the tremors, causing anything on the surface to sink. When the tremors stop, the ground become solid again and anything that has sunk — cars, houses, people— becomes trapped in the rock.

A more familiar example occurs in packets of mixed nuts where anecdotal evidence has long suggested that the largest nuts, Brazil nuts, rise to the top. A similar phenomena occurs in packets of breakfast cereal, such as muesli.

The effect turns out to be complex. It depends on many parameters, such as the ratio of large particles to small ones, their relative densities, the frequency vibrations and so on. But when there are only a few large particles and all are of equal density, shaking creates spaces beneath the large particles that small ones can fall into. This has the effect of making the large particles rise.

The question that Matsumura and co address is whether this process could have somehow caused large boulders to float to the surface of asteroids. Their thinking is that collisions between asteroids might create the kind of vibrations that allow large boulders to rise, in exactly the same way as Brazil nuts in packets of mixed nuts.

Of course, asteroid dynamics are different in one very important way from those that mixed nuts experience: their gravity is much weaker.

So an important question is whether the same dynamics can occur under low gravity conditions as under Earth’s gravity. To find out, Matsumura and co conducted number of simulations in which they filled a long cylindrical container with 1800 small particles and one large one at the bottom with three times the diameter. All the particles had the density of aluminium.

They simulated the gravity of Earth, the Moon and several asteroids including Eros and Itokawa. And they show that if they reduce the strength of the vibrations to match the level of gravity, then the Brazil nut effect occurs in all cases, provided that the oscillation speeds are above some critical threshold that turns the granular material from solid-like to fluid-like.

The key question, of course, is whether the oscillations in these experiments are similar in strength and frequency to those that occur when rocks collide with asteroids, such as Eros and Itokawa. Matsumura and co say that the simulations show this is exactly the case. “In both cases, we expect that the critical Brazil nut effect oscillation speeds are comparable to the seismic speeds that can create craters,” they say.

And they calculate that large boulders should rise to the surface of Itokawa or Eros in a few hours or about a day. “This implies that unless the seismic shaking lasts for more than a couple of hours, one impact might not be enough for a large block to rise to the surface, and that multiple impacts would be necessary to change the surface signiﬁcantly ,” say Matsumura and co.

That’s a fascinating result. It suggests that the distribution of boulders on the surface of asteroids is the result of the Brazil nut effect that occurs during collisions that shake these bodies. And that the distribution we see today is the result of the many collisions that have occurred throughout the history of the Solar system.

So the next time you open a packet of mixed nuts, remember that the same forces are at work in the heavens above.